1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a semiconductor device with metal gate.
2. Description of the Prior Art
In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers.
Typically, threshold voltage in conventional planar metal gate transistors is adjusted by the means of ion implantation. With the trend in the industry being towards scaling down the size of the metal oxide semiconductor transistors (MOS), three-dimensional or non-planar transistor technology, such as fin field effect transistor technology (FinFET) has been developed to replace planar MOS transistors. Nevertheless, when electrical field applied onto a dielectric material exceeds a threshold value, a sudden increase in the electrical current passing through the dielectric material would easily induce a time-dependent dielectric breakdown (TDDB) issue. Hence, how to resolve this issue in today's FinFET architecture has become an important task in this field.